In a portable telephone with a built-in digital camera, an imaging lens is mounted on a printed circuit board. As a method for mounting an imaging lens on a printed circuit board, a reflow soldering processing is used. Hereafter the reflow soldering may simply be called “reflow”. Reflow processing is a method for soldering an electronic component by placing a solder ball in advance at a location where an electronic component is connected, placing the electronic component there, heating to meld the solder ball, then cooling the solder down.
Generally in mass production steps, a reflow step for performing reflow processing is used as a method for mounting electronic elements or such components as an imaging lens on a printed circuit board. If the reflow step is used, the mounting cost of components on a printed circuit board can be decreased, and manufacturing quality can be maintained at a predetermined level.
In the reflow step of the manufacturing steps of a portable telephone comprising an imaging lens, not only are electronic components arranged at predetermined positions on a printed circuit board, but also the imaging lens itself or a socket for installing the imaging lens is disposed on the printed circuit board.
The imaging lens installed in portable telephones are largely made of plastic in order to decrease the manufacturing cost, and to insure lens performance. Therefore a heat resistant socket component is used for installing an imaging lens in order to prevent thermal deformation of the imaging lens in a high temperature environment, which makes it impossible to maintain optical performance thereof.
In other words, in the reflow step, a heat resistant socket component for installing an imaging lens is mounted on the printed circuit board of the portable telephone, so that the imaging lens is not exposed to high temperature in the reflow step (e.g. see Patent Documents 1 to 3). However, using a heat resistant socket component for installing an imaging lens makes the manufacturing steps complicated, and increases the manufacturing cost, including the cost of this heat resistant socket.
A recent demand is that the optical performance of an imaging lens installed in a portable telephone does not deteriorate even if the portable telephone itself is placed in a high temperature environment at 150° C. or higher, considering the case of the portable telephone being left in an automobile which temporarily becomes a high temperature environment. A conventional imaging lens made of plastic material cannot meet this demand.
In order to implement an imaging lens of which optical performance is maintained even in a high temperature environment, forming an imaging lens using a high softening temperature mold glass material is possible (e.g. see Patent Document 4). According to this, the deterioration of optical performance of an imaging lens in a high temperature environment can be avoided, but at the moment, an imaging lens made of mold glass material is not very popular, because the manufacturing cost is very high.
In addition to the above mentioned thermal characteristics, an imaging lens installed in a portable telephone must satisfy the following conditions related to optical characteristics. One condition is that the optical length, which is defined as a distance from an entrance plane at an object side to an image formation plane (also called “image sensing plane”) of the imaging lens, must be short. In other words, when a lens is designed, the ratio of the optical length to the composite focal distance of the imaging lens must be minimized.
In the case of a portable telephone, for example, this optical length must at least be shorter than the thickness of the portable telephone unit. On the other hand, a back focus, which is defined as a distance from the outgoing plane at the image side to the image sensing plane of the imaging lens, should be as long as possible. In other words, when the lens is designed, the ratio of the back focus to the focal distance must be maximized. This is because such components as a filter and a cover glass must be inserted between the imaging lens and the image sensing plane.
In addition to this, it is naturally demanded for the imaging lens that various aberrations are corrected to be small enough that the distortion of the image is not visually recognized, and that the integration density of the image sensing elements in minimal units (also called “pixels”), which are arranged in a matrix on the light receiving plane of a CCD (Charge Coupled Device) image sensor, is sufficiently satisfied. In other words, various aberrations of the imaging lens must be well corrected. Hereafter an image, of which various aberrations are well corrected, may be called a “good image”.
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-121079 (U.S. Pat. No. 3,799,615)
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-328474 (U.S. Pat. No. 3,915,733)
Patent Document 3: Japanese Patent Application Laid-Open No. 2004-063787 (U.S. Pat. No. 3,755,149)
Patent Document 4: Japanese Patent Application Laid-Open No. 2005-067999